Repurchase Arrangements And Similar Borrowings, Including Related Hedging Activity (Narrative) (Detail) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2012 Year	Dec. 31, 2011	Dec. 31, 2010 Year
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Swap agreements average expiration period (in months)	17 months
Notional amount of swap agreements	$ 3,700,000,000		$ 100,000,000
Average fixed rates of interest rate swap agreements	0.80%		4.09%
Term of LIBOR interest rate agreement (in years)	20		20
Unrealized losses on derivatives held as cash flow hedges	37,400,000	30,200,000
Interest Rate Swap
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Swap agreements hedging short-term interest rates	$ 700,000,000
Minimum
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Maturity period of repurchase arrangements (in days)	30
Maximum
Repurchase Arrangements And Similar Borrowings, Including Interest Rate Hedging Activity [Line Items]
Maturity period of repurchase arrangements (in days)	90